It has been a difficult month for the family of slain MIT police officer Sean Collier so when a university official contacted them asking: "Would you like to meet the president?" his grieving mother's response was, "The president of what?"

"The United States, ma'm,'' was the answer she got.

President Obama quietly invited Collier's family to a private meeting in Boston and personally thanked Collier's mother, Kelly Rogers, his stepfather, Joe, and his siblings for the sacrifice that Collier made for the nation on April 18.

Collier, 27, was assassinated as he sat in his patrol car by the two accused Boston Marathon bombers, police believe.

"He was so sincere. He expressed his condolences and told us how much he appreciates Sean's sacrifice and ours,'' Kelly Rogers told ABC News during an interview at the family's Massachusetts home. "The president made us all feel like he really did understand our loss."

"It was very nice. He hugged every one of us,'' said Collier's sister, Jennifer Lemmerman, 30.

The meeting took place at the Reggie Lewis Center in Roxbury where Obama was campaigning for Democratic Senate candidate Edward Markey.

Collier was 28 minutes away from finishing his shift when he parked on a courtyard that the university had received noise complaints about. He had also studied the photographs released roughly five hours earlier of the two men wanted in connection with the Marathon bombings.

Just after 10:30 p.m., a security surveillance video shows what appears to be the bombing suspects, Tamerlan and Dzhokhar Tsarnaev, crouch up behind Collier's cruiser.

Without uttering a word, one of the brothers believed to be Tamerlan Tsarnaev ripped open the driver's door and shot Collier five times, several police sources have told ABC News. At the passenger door Dzhokhar Tsarnaev tried to remove Collier's service weapon, but could not manage to maneuver the safety retention lock, officials said.

The video shows passersby approaching and the brothers fled. Collier was found dead by an MIT colleague. Collier's slaying took less than 30 seconds.

The Tsarnaev brothers went on to carjack a man and used his vehicle to flee to a street corner in Watertown where a gun and bomb battle with police ensued.

Less than 24-hours later, Tamerlan Tsarnaev, 26, was dead. Dzhokhar Tsarnaev, 19, was found hiding on a boat in Watertown.

MBTA Officer Richard Donahue, Sean Collier's police academy classmate, was in critical condition from a wound suffered in the gunbattle. A third MBTA police officer was grazed by a bullet.

Still, a part of Rogers was trying to hold onto hope. But in the emergency room of Massachusetts General Hospital she knew by the presence of so many police officers, men who loved and respected her son, that Sean was gone. "I thought, there goes my hope," she said.

But hope is what Collier's tight-knit family is holding onto now – especially the hope that their meeting with President Obama could spark a movement to recognize the sacrifice of first responders who die in the line of duty with a national federal holiday.

The movement is the brainchild of Sean's closest friend, his younger brother Andrew, 25. He is collecting signatures at 2MAKEACHANGE.ORG to prompt legislation.

"We have Memorial Day for families of our military,'' Lemmerman told ABC News. "Andrew wants to have a national holiday for families like ours."